Our long-term objective is to determine the mechanisms that regulate cellular growth and neoplastic transformation in breast tissue. Our current studies focus on how the tumor susceptibility gene 101 (TSG101) prevents tumorigenesis in mammary epithelial cells. TSG101 has been recently identified as an important factor for growth restriction and maintenance of genome stability in murine cell lines. The protein encoded by this gene has a very high similarity between human and rodents. Although chromosomal deletions of TSG 101 are rare events, aberrant transcripts are observed quite frequently in human cancers. Despite its important role for the maintenance of genome integrity in cultured cells, the biological function of TSG101 in vivo is largely unknown. The specific aims of this proposal are to determine the role of TSG101 during normal development and tumorigenesis in vivo. We use genetically engineered mice that carry somatic mutation for TSG101 in mammary tissue. We will investigate whether TSG101 has characteristics of a tumor suppressor gene or mammary tissue. The critical domains within TSG101 that mediate its role during tumorigenesis will be determined by replacing the endogenous TSG1021 protein with shorter with shorter TSG101 mutants that lack specific parts of the protein. Finally, we will investigate the suggested role of TSG101 as a co-factor for the transcriptional repression of targeted by DNA methyltransferase 1. TSG101 might function in various different pathways that are important for growth, differentiation, and cancer progression. Thus, investigating the regulation of TSG101 expression and its biological function will provide us with new insights that will allow us to intercept oncogenic pathways, that require adequate levels of this protein to function normally.